In the manufacturing process of a shoe, a shoe portion to be attached to other portion(s) of the shoe is applied with glue. Taiwanese Utility Model Patent No. M503948 discloses a conventional approach of inspecting the applied glue which includes step of determining a spreading area of the glue applied on the shoe portion by means of detecting light beams passing through the glue. However, the conventional approach cannot be utilized to determine variation in the amount of glue over an area of the shoe portion.